[Mp4-tech] frame_num, h.264

[Sastry_KRK]

hello experts, 
frame_num may not take a value equal to prevREfframeNum, in addition to two
other conditions ,  the following condition is true
" the current picture and the preceding reference picture are reference
fields having opposite parity "
suppose only frames support is there and no support for fields, the
frame_num will never be equal to prevREfframeNum.
could any one explain this behavior  
regards
KRK Sastry
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